PROBLEMS OF ASTROPHYSICS 453 



thought it remarkable that nitrogen should have two or three char- 

 acteristic spectra, or that a metal should have a spark spectrum and 

 an arc spectrum. We are now confronted with the potent fact that 

 an element may have a variety of spectra, depending upon the nature 

 and the intensity of the forces employed in rendering it luminous. 1 

 But for most cases these involve only moderate variations in the 

 relative intensities of spectral lines. The complications which threaten 

 to result therefrom are more apparent than real. The multiplicity 

 of spectral reactions promises to be a powerful aid to analysis, by 

 supplying a more exact key to the conditions in the celestial light 

 source which produce the observed effects. 



For many years following the application of the spectroscope to 

 celestial problems it was supposed that a continuous spectrum must 

 indicate incandescent solid or liquid matter. The situation is not 

 so simple as this. Some gases radiating under high pressures give 

 spectra apparently continuous. 



The effect of increasing temperature conditions on certain spectra 

 has long been well known. Certain lines are enhanced in relative 

 brilliancy when we pass from the temperature of the arc to that of 

 the high-tension spark, and vice versa ; 2 but it seems certain that, 

 within measurable limits, the positions of the lines do not change 

 under this influence. 



Humphreys and Mohler 3 have proved that the spectral lines are 

 shifted by pressure; toward the red with increasing pressure in the 

 atmosphere surrounding the arc. It is not difficult to see the bear- 

 ing of this discovery upon astrophysical inquiry. Some subjects are 

 made more complex; but the hope is held out that eventually we 

 may detect these indications of pressure, differentially, in the brighter 

 stars. 



It is also known that the spectra of some elements are altered by 

 the presence of other elements, 4 but the extent and character of the 

 induced changes are little understood. As the chemical elements are 

 never found alone in celestial bodies, the serious consequences of this 

 effect must be evident. 



The temperature in glowing Plucker tubes is of great interest, from 

 its bearing upon the probable temperatures of nebulae, the aurorse, and 

 other bright-line phenomena of a diffuse nature. It is not certain 

 that direct observation by any thermometric device can deal with the 

 problem. The measures thus far attempted have assigned tempera- 

 tures but a few degrees higher than that of the environment. These 



1 Kayser's Handbuch der Spectroscopie, 11, 222-286. 



2 Berlin, Berichte, 1894,257-258; Astronomy and Astro-Physics,xm, 660-662; 

 Astrophysical Journal, xvn, 270; and many others. 



3 Astrophysical Journal, in, 114; iv, 175; vi, 169. 



* Lewis, Astrophysical Journal, x, 137; Nutting, Bulletin Bureau of Standards, 

 i,77. 



